PFE Chapter 1, Time value of money page 54
• Put all the variables which are important (the fashionable jargon is “value drivers”) at the
top of your spreadsheet. In the “Saving for College” spreadsheet of page000, the three
value drivers—the interest rate, the annual deposit, and the annual cost of college—are in
the top left-hand corner of the spreadsheet:
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ABCDE
Interest rate 8%
Annual deposit 6,227.78
Annual cost of college 20,000
Birthday
In bank on birthday,
before
deposit/withdrawal
Deposit
or withdrawal
at begin. of year
Total
End of year
with interest
10 0.00 6,227.78 6,227.78 6,726.00
11 6,726.00 6,227.78 12,953.77 13,990.08
12 13,990.08 6,227.78 20,217.85 21,835.28
13 21,835.28 6,227.78 28,063.06 30,308.10
14 30,308.10 6,227.78 36,535.88 39,458.75
15 39,458.75 6,227.78 45,686.52 49,341.45
16 49,341.45 6,227.78 55,569.22 60,014.76
17 60,014.76 6,227.78 66,242.54 71,541.94
18 71,541.94 -20,000.00 51,541.94 55,665.29
19 55,665.29 -20,000.00 35,665.29 38,518.52
20 38,518.52 -20,000.00 18,518.52 20,000.00
21 20,000.00 -20,000.00 0.00 0.00
NPV of all payments 0.0000 <-- =NPV(B2,C8:C18)+C7
SAVING FOR COLLEGE
•
Never use a number where a formula will also work. Using formulas instead of “hard-
wiring” numbers means that when you change a parameter value, the rest of the
spreadsheet changes appropriately. As an example—cell C20 in the above spreadsheet
contains the formula
=NPV(B2,C8:C18)+C7. We could have written this as
=NPV(8%,C8:C18)+C7. But this means that changing the entry in cell B2 won’t go
through the whole model.
•
Avoid the use of blank columns to accommodate cell “spillovers.” Here’s an example of
a potentially bad model:
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ABC
Interest rate 6%